The invention relates to a device for the detection of electromagnetic radiations, and moreover to its visualization in analogue form.
Although the invention which follows is more particularly described with regard to the detection of infrared radiations, the present invention is also applicable to the field of the detection of visible or ultraviolet radiations.
The device of the invention implements thermal detectors of microbolometric type. Specifically, detectors of this type can operate at ambient temperature, that is to say without the need for cooling, in contradistinction to the device of the quantum detectors type, which directly convert the energy of the radiation captured into free electrical carriers.
This type of uncooled detector employs the variation of a property of one of the materials of which they consist as a function of temperature. Within the framework of the implementation of bolometric detectors, this property is the resistivity of the material. In a known manner, such an uncooled detector associated for each photosite or pixel comprises:    means of absorption of the radiation, for the conversion of the latter into heat;    means of thermal insulation of the detector, allowing the latter to warm up;    thermometry means, which, within the framework of a bolometric detector use a resistive element varying with temperature; and    means of reading electrical signals emanating from the thermometry means, the said reading means integrating a component for making contact and for transporting the signal from the bolometric material to a component for utilizing the signal, in general of microelectronic kind.
Within the framework of infrared imaging, the detectors employed are generally embodied in the form of arrays of elementary detectors laid on a substrate, usually consisting of silicon.
The implementation of such bolometric detection devices is not without the creation of problems of a technical nature.
Firstly, the performance of such an uncooled bolometric detector depends on various factors, including obviously the bolometric material employed.
Next, in order for these bolometric detectors to be able to operate, they need to be biased by a biasing current. Now, over an array of microbolometers, a dispersion is observed in the value of the nominal resistance of the various detectors, biased to the same constant voltage. A consequence of this dispersion is that the current for biasing the microbolometers is not uniform. Thus, a first solution to this problem has consisted in undertaking a global baselining, per column of pixels, carried out by means of a thermally isolated microbolometer.
Thus, represented within FIG. 1 is a schematic pixel (1) implementing a bolometric detector (2), biased by means of a voltage-driven transistor (3). The resistivity of the detector (2) is proportional to the amount of radiation that it receives, this being manifested as a variation in its bias current. This current emanates from a first baselining, the so-called global baselining, by means of a thermally isolated microbolometer (8) subjected to a constant baselining voltage V/Baselining. The expression thermally isolated is understood to mean a microbolometer whose resistivity is constant and independent of the radiation sensed. One also speaks of a blind microbolometer.
Also represented within this figure is the line (12) for row-wise selection of the pixel considered, acting on a switch (4), allowing the routing of the current resulting from the bolometric detector (2) to the level of a CTIA (11) (standing for the expression “Capacitive TransImpedance Amplifier”), charged with amplifying the said signal and with converting it into voltage via an integration capacitor (15), before its utilization for its restitution in the form in particular of video signals.
This global baselining induces the removal of the considerable dispersions of the bias current of the microbolometers (2), induced by the dispersions in the resistivity of the said detectors.
A consequence of the thermally isolated microbolometer (8) employed is that the current integrated by the reading circuit depends to the greatest possible extent on the infrared radiation or on the radiation detected, and not on the bias current.
Nonetheless, this so-called global baselining alone is not sufficient to obtain a satisfactory output signal. Specifically, given the mode of manufacture of detectors, bolometric detectors in particular, they are observed to exhibit dispersed values of resistance.
Thus, for a specified radiation and a specified integration capacitance, several microbolometers may reach the saturation zone situated outside the range of the dynamic swing of the input stage of the reading circuit. It has therefore been proposed that the global baselining device be supplemented with an additional device, called adaptive baselining, specific to each of the pixels of the array of the detection circuit, this amounting to improving the dynamic swing of the input stage.
This adaptive baselining function can be carried out by acting on the gate voltage of the microbolometer bias transistor VFID (see for example document U.S. Pat. No. 6,028,309).
In order to provide for this adaptive baselining function, it has also been proposed that for each pixel there will also be added a programmable current generator (9), which acts in parallel with the global bias current, generating a current for subtraction from the signal generated by the detector, as a function of the dispersion inherent to the pixels considered relative to a reference signal, and stored in an associated memory. In this case, a digital cue representative of the dispersion value for each of the pixels is stored outside the reading and detection circuit.
This pixel-wise adaptive baselining is carried out during the integration phase, that is to say the phase of acquisition of the image, by means of a programmable current source, also dubbed “baselining DAC” (DAC=digital analogue converter). The resolution of the DAC being 3 bits, it is necessary to store for each pixel the binary baselining value on 3 mapped bits.
This value is determined during a calibration phase that proceeds in the following manner:    a well-determined reference phase is presented to the array of detectors;    the baselining data provided to the circuit before integration are such that no baselining current is injected;    the reading and the analogue/digital conversion of the video signal emanating from this image are carried out by virtue of an analogue digital converter;    the 3 most significant bits of each pixel are stored in an external memory outside the reading circuit.
Thus, during the nominal operation of the circuit, each phase of integration of a row of the array is preceded by a phase of acquisition of the baselining data stored in the external memory for the row of pixels considered. The transferring of the data between the external memory and the reading circuit is performed sequentially on three digital inputs, that is to say that the 3 bits are programmed by bit/pixel of one and the same row.
If the implementation of such adaptive baselining proves satisfactory at the level of the quality of the signals thus detected, transcribed into analogue form, on the other hand, and this emerges very distinctly from the foregoing, it requires the implementation of an external memory, associated with the detection/reading circuit, thus to some extent complicating the electronics of the reading and detection circuit.
Furthermore, according to this process, the reading of the digital information representative of the three most significant bits specific to each pixel is performed during analogue integration by the said detectors, generating additional noise at the level of the reading circuit. This too emerges very clearly from the time chart representative of the baselining reading of the devices of the prior art, and represented in FIG. (2). This time chart corresponds to the schematic illustrated in FIG. 3. Thus, the three external-baselining data bits, previously stored in the external memory during the calibration phase, are transmitted sequentially to the reading circuit according to a tempo imposed by the pixel clock SYP. These data are stored temporarily in an internal buffer memory, called LATCH, which holds the baselining information for a complete row throughout the duration of integration. It therefore emerges that from a time chart point of view, the reading of the baselining data of row n is performed during the integration of row n−1 in a register with serial input and parallel output. At the end of the integration phase, the toggling of the synchronization signal to the high state (line SYL) triggers the transfer of the baselining data into the buffer memory LATCH, and the integration of row n can then commence, upon the toggling of the said synchronization signal SYL to the low state. There is therefore no immunity between the analogue processing and the digital processing of the signals at the level of the pixel and of the column to which it belongs, in particular in the memory writing phase.
A consequence of this is a degradation in the noise performance of the reading circuit, which has to remain compatible with the detector's own characteristic (lying between 250 μV and 500 μV).
In the schematic of FIG. 3, the external analogue digital converter (ADC) codes the video signal on three bits during the calibration phase. In this configuration, it is the electronics for driving the circuit that provide for the write/read management of the baselining data at the level of the external memory and of the reading circuit.